The present invention relates to an illumination apparatus, which has an optical member for efficiently condensing luminous flux emerging from a light source such as a light-emitting diode to a required irradiation area, and an image-taking apparatus, which has the illumination apparatus.
Conventionally, a number of illumination apparatuses employing a light-emitting diode (hereinafter abbreviated to “LED”) as a light source have been put into practical use in which the LED is provided with a light-condensing function. The LED of this type is formed to have a light-emitting diode chip fixed inside a metal cup serving as a reflecting mirror and a dome-shaped transparent resin having a light-condensing function formed on the front surface (the surface on the emergence side) of the metal cup.
The LED of the aforementioned structure can provide different degrees of light condensing by changing the diameter or curvature of the portion of the transparent resin. An apparatus which employs such an LED has been proposed (for example, Japanese Patent Laid-Open No. 10-21703).
On the other hand, another proposed optical system has an LED as a light source without any light-condensing function and has a convex lens disposed on the light irradiation side of the light source and condensing luminous flux emerging from the light source (for example, Japanese Patent Laid-Open No. 2000-89318).
Yet another proposed optical system has an LED as a light source without any light-condensing function and has a prism which is used to reflect luminous flux emerging from the light source and to send the luminous flux toward a particular irradiation area (for example, Japanese Patent Laid-Open No. 10-164315).
Conventionally, the dome type LED having the light-condensing function has an optically optimal arrangement for the light-emitting diode chip serving as the light source to realize an optical system with a high level of light condensing, although it often depends on the optical specifications of the LED. Specifically, no problem occurs when it is possible to select an LED which can irradiate light to an optimal irradiation area required for an optical device on which the LED is mounted, but the irradiation area of an LED is not always identical to the optimal irradiation area of the optical device.
If the existing LED having the optical characteristic (irradiation angle) described above is used to attempt to irradiate light to the optimal irradiation area of an optical device, another optical system (optical member) is required as in the structure shown in Japanese Patent Laid-Open No. 10-21703. In this case, the addition of the other optical system increases the size or cost of the resulting illumination apparatus.
On the other hand, the LED of a surface emitting type which has no lens formed integrally on the front surface generally uses a convex lens disposed on the side of light irradiation to condense light as shown in Japanese Patent Laid-Open No. 2000-89318. When the convex lens is used to condense light, the focus length of the lens can be set appropriately to illuminate a particular area including an irradiation optical axis within the light irradiation area, but space is left between the LED as the light source and the convex lens.
Thus, part of luminous flux emerging from the LED escapes from the space to reduce the irradiation efficiency of the illumination light.
In the light-condensing optical system which uses the convex lens to utilize only refracted light, while the central area (the area near the irradiation optical axis) of the light irradiation area can be irradiated relatively brightly, sufficient illuminance is difficult to ensure in peripheral areas other than the central area. It is thus difficult to provide illumination of uniform brightness over the entire required irradiation area.
Optical systems which use the LED of the surface-emitting type having no lens formed integrally therewith to change the direction of light emergence typically include the illumination optical system which has a prism surface and a reflecting surface arranged on the front surface of the light source (on the side of light irradiation) to send light in a predetermined direction while the light is redirected and condensed, as shown in Japanese Patent Laid-Open No. 10-164315.
In the aforementioned illumination optical system, however, luminous flux near the irradiation optical axis is merely condensed sequentially while the emergence direction thereof is changed by refraction or reflection, and the optical system is not formed to improve the light-condensing efficiency by making use of refraction and reflection.
Since luminous flux deviated from the irradiation optical axis may travel in a direction different from a desired direction (direction in design), the optical system is not preferable in terms of efficient and uniform irradiation of luminous flux emerging from the LED to a particular area on the irradiation surface.
In this manner, the conventional illumination optical system using the LED is formed to condense luminous flux by utilizing one of the two optical effects, that is, refraction or reflection. In such a structure, however, the luminous flux emerging from the LED cannot be necessarily used efficiently as described above, and the illuminance distribution in the illumination area is not always uniform.
In other words, when the single lens is used to condense luminous flux emerging from the small light source such as the LED, the central area near the irradiation optical axis is irradiated brightly, but peripheral areas other than the central area are extremely dark in a light distribution characteristic typically shown.
When a light source having LED elements of a plurality of colors packaged together for a single condenser lens is used, these LED elements are arranged at different positions and it is difficult to match the irradiation areas of the respective LED elements. This results in unevenly distributed light and uneven color.
It is contemplated that each surface of an optical member is made as a diffusing surface or a material of the optical member contains a component for diffusing luminous flux as a measure to prevent the aforementioned unevenly distributed light and uneven color. When the diffusing component is contained, however, it is generally known that the irradiation efficiency is extremely reduced, and an optical system cannot be formed with high irradiation efficiency. It is thus preferable to employ a structure in which luminous flux from the light source is sent by using only the optical action such as refraction and reflection.